Numerical investigation of the interactions between a laser-generated bubble and a particle near a solid wall

The interactions between the bubbles and the particles near structures are important issues for the applications of the cavitation in the fluid machinery.To stu...     

空泡在固壁附近潰灭的数值模拟

实验表明,静止流场中的近壁空泡在泡内外压差的驱动下将发生变形收缩,直至完全溃灭消失。在这一过程中,伴随着空泡形状的变化,流场中出现多个高压脉冲和高速射流。本文通过直接求解原始变量的N-S方程,用VOF方法计算气液两相交界面的运动过程,对这一现象进行了数值模拟和分析。结果表明理论分析和实验现象相吻合。     

Investigation of cavitation bubble collapse near rigid boundary by lattice Boltzmann method

The dynamics of the bubble collapse near a rigid boundary is a fundamental issue for the bubble collapse application and prevention. In this paper, the bubble collapse is modeled by adopting the lattice Boltzmann method(LBM) and is verified, and then the dynamic characteristics of the collapsing bubble with the second collapse is investigated. The widely used Shan-Chen model in the LBM multiphase community is modified by coupling with the Carnahan-Starling equation of state(C-S EOS) and the exact difference method(EDM) for the forcing term treatment. The simulation results of the bubble profile evolution by the LBM are in excellent agreements with the theoretical and experimental results. From the two-dimensional pressure field evolution, the dynamic characteristics of the different parts during the bubble collapse stage are studied. The role of the second collapse in the rigid boundary damage is discussed, and the impeding effect between two collapses is demonstrated.     

Experimental study of the interaction between the spark-induced cavitation bubble and the air bubble

Experiments are carried out by using high-speed photography to investigate the interaction between the spark-generated cavitation bubble and the air bubble in its surrounding fluid. Three problems are discussed in detail： the impact of the air bubble upon the development of the cavitation bubble, the evolution of the air bubble under the influence of the cavitation bubble, and the change of the fluid pressure during the development of a micro jet of the cavitation bubble. Based on the experimental results, under the condition of no air bubble present, the lifetime of the cavitation bubble from expansion to contraction increases with the increase of the maximum radius. On the other hand, when there is an air bubble present, different sized cavitation bubbles have similarity with one another generally in terms of the lifetime from expansion to contraction, which does not depend on the maximum radius. Also, with the presence of an air bubble, the lifetime of the smaller cavitation bubble is extended while that of the bigger ones reduced. Furthermore, it is shown in the experiment that the low pressure formed in the opposite direction to the cavitation bubble micro jet makes the air bubble in the low pressure area being stretched into a steplike shape.     

Determination of the shedding frequency of cavitation cloud in a submerged cavitation jet based on high-speed photography images

To accurately determine the shedding frequency of the cavitation cloud in a submerged cavitation jet,the spectral analysis and the proper orthogonal decomposition(POD)for high-speed photography images are performed.The spectrums of 6 different kinds of image signals(the area-averaged gray level,the line-averaged gray level,the point gray level,the cavitation length,width,and area)are calculated and compared.The line-averaged gray level is found to be optimal in determining the shedding frequency but an accurate frequency can only be obtained in the stable-frequency zone where the cavitation cloud sheds.In repeated experiments,the plateau-shape distribution of the main frequency is established with a deviation of 10.8%.A revised Reynolds number Re'is defined and the shedding frequency can be correlated to Re'by a power law when the cavitation number is less than 0.02.This relationship is validated by the experimental data in literature.The first mode of the POD characterizes the ensemble-average of the cavitation cloud while the second mode is the major part of the cavitation cloud transient components.The modes 2-5 are organized in pairs,which confirms the periodic feature of the cavitation cloud in the submerged cavitation jet.Near the nozzle exit,the modes 2-5 are symmetrically distributed in the jet shear layer.The shedding frequency of the cloud cavitation can also be precisely determined by performing the spectral analysis of the weighting coefficients of the mode 2.This paper shows that the two parameters,namely,the line-averaged gray level and the weighting coefficients of the mode 2,can be confidently used to calculate the shedding frequency of the cavitation cloud in a submerged cavitation jet.     

静水中大雷诺数球形颗粒变加速沉降的理论分析与试验

基于单颗粒泥沙沉降的力学机理,对静水中大雷诺数球形颗粒沉降过程中的受力状况进行分析,推导出变加速沉降运动过程的瞬时速度、沉降历时及沉降位移公式。试验数据与所推导公式的计算结果对比分析表明:计算位移与实测值相对误差都小于5%,大部分小于4%;瞬时沉速计算值与试验值相对误差都在7%以内,大部分小于5%,计算值与实测值吻合良好。     

灌注桩钻孔塌孔实测及孔壁稳定性评价

以沿海地区某大型工程钻孔灌注桩地基处理为背景,在典型区域灌注桩成孔质量实测的基础上,提出了一种新的孔壁稳定或坍塌程度评价参数——塌孔率,建立了塌孔孔径划分标准和坍塌体积计算模型,对塌孔体积与砂层厚度、钻孔深度的相关性进行了回归分析。对塌孔率与砂层平均标贯数、砂土不均匀系数及砂土平均粒径的相关性进行了回归分析。研究结果表明:砂土层是灌注桩钻孔时孔壁稳定的关键;用塌孔率来评价孔壁稳定和坍塌程度是可行且适用的;塌孔体积与砂层厚度、钻孔深度密切相关;砂层平均标贯数、砂土不均匀系数及砂土平均粒径是灌注桩施工孔壁稳定性及塌孔风险评价的重要指标。     

汽液黏滞系数对空化泡溃灭过程影响的数值模拟

利用格子玻尔兹曼方法(LBM)伪势模型模拟了近壁区空化泡溃灭过程,分析了不同汽相黏滞系数和液相黏滞系数对空化泡溃灭过程中的最大微射流流速、最大溃灭压力、最大溃灭时间的影响。结果表明,相同液相黏滞系数条件下,改变汽相黏滞系数,空化泡溃灭时产生的最大微射流流速、最大溃灭压力和最大溃灭时间不变;保持汽相黏滞系数不变,空化泡溃灭时最大溃灭压力、最大微射流流速均随着液相黏滞系数的增大而减小,但最大溃灭时间随着液相黏滞系数的增大而增大。     

A thermal-hydrodynamic coupling method for simulating the interplay between bubble departure and wall temperature variation in nucleate boiling

A thermal-hydrodynamic coupling model is used to investigate the interplay between the wall temperature distribution and the bubble departure during the nucleat...     

An experimental investigation of transient cavitation control on a hydrofoil using hemispherical vortex generators

Unsteady cavitation causes noise,damage,and performance decline in the marine engineering and fluid machinery systems.Therefore,finding a method to control the ...     

振荡流边界层中颗粒受力的三维格子玻耳兹曼模拟

应用格子玻耳兹曼方法对振荡流边界层中单个颗粒在近壁面处的受力进行了三维模拟,计算结果与已有数值模拟结果(雷诺数为300)和实验结果(雷诺数为2000)进行了比较,吻合均较好。计算结果表明,当雷诺数为300时,在振荡流做正反向运动时力系数的幅值变化过程相同,横向涡量的空间分布具有对称性。雷诺数为2000时,紊动增强使力系数在不同流动方向时的变化有所差异,而且横向涡量的分布呈现非对称性,这时颗粒受到横向力作用。     

双孤立波直墙爬高的实验研究

海啸波在近海传播过程中有可能会演化成若干个孤立波组成的波列.为了研究孤立波波列对海岸结构物的作用机理,本文在波浪水槽中进行了双孤立波的直墙爬高实验.通过控制推板式造波机的运动,有效地实现了不同波峰间距双孤立波的造波方法.在复演并验证单孤立波直墙爬高的基础上,完成了不同相对波高、相对波峰间距的双孤立波爬高系列实验,给出了双孤立波平底传播及直墙爬高过程的波面时间序列及爬高过程水线运动时间序列.实验结果表明:对两个等波高的双孤立波而言,当相对波峰间距降到某临界值时,跟随在后孤立波的爬高放大系数将小于先导孤立波的爬高放大系数;当先导孤立波的波高大于跟随孤立波的波高,先导孤立波将更加显著地抑制对跟随孤立波的爬高过程,使得后者的最高爬放大系数显著减小;若在直墙爬高前两个波已明显重叠,即呈双峰瞬态波形,爬高放大系数不会大于具有相同波高的单孤立波的爬高放大系数.     

防渗墙刚性混凝土单轴及三轴受压的本构关系试验研究

采用简易σ－ε测试系统和日本产动液压伺服铡性试验机进行了防渗墙用刚性混凝土的单轴及三轴受压的全应力－应变试验，并分析了刚性混凝土在受压时不同阶段的破坏行为。成果表明：试验所用的刚性混凝土弹模值较低，比例极限较高，峰值应变较大；低围压的三轴状态下刚性混凝土的轴压强度和弹模随围压增加而增大，但弹强比则相反，这种刚性混凝土作防渗墙墙体材料对周围介质的变形有较好的适应能力，全应力－应变曲线对防渗墙的非线性     

水平井筒变质量分散泡状流压降的理论与实验研究

由于水平井筒和常规水平管道中气液两相流动的相似和差别，可以预知常规水平管流的压降计算方法对于井筒流动来说就需要进行修正或扩展。本文对气、液两相分别应用质量守恒方程和动量守恒方程，考虑管壁存在人流或出流对于分层流流型压降的影响，得到水平井筒气液两相变质量流动分散泡状流流型的压降计算方法。同时，设计并建立了水平井筒流体流动模拟实验装置，在轴向为气液两相流动的前提下分别进行了上管壁单孔眼注入和下管壁单孔眼注入的压降实验研究，获得了大量的实验数据。实验数据和理论计算结果吻合很好，这表明该计算方法具有实际应用价值。     

渡槽薄壁结构弹塑性动力分析模型及试验验证

本文提出了渡槽薄壁结构弹塑性动力分析模型，该模型集渡槽薄壁结构线弹性模型和非线性多弹簧模型为一体，并充分考虑了薄壁结构截面两向弯矩和轴向变形对单元的动力特性影响，应用多弹簧等效关系模拟结构塑性铰区的非线性滞回特性。通过预应力钢筋混凝土渡槽槽身以及高强混凝土渡槽支架模型的低周反复荷载拟静力试验，研究了渡槽槽身以及支架的非线性荷载-位移滞回特性。应用本文提出的弹塑性动力分析模型对拟静力试验过程中的荷载-位移滞回曲线进行了数值模拟计算。计算结果表明，模拟数值解与模型试验所得滞回曲线符合良好，从而验证了本文渡槽薄壁结构弹塑性动力分析模型的正确性。     

Origins of filter effluent particles: experimental study of particle deposition and detachment.

This paper investigates the relative roles of particle deposition and detachment in controlling the origin of filter effluent particles. A conceptual mathematical model was developed and laboratory-scale experiments were conducted. Laboratory experiments were performed using three sizes of fluorescent microspheres (FMs), to determine the fraction of filter effluent particles that are filter influent particles that were never removed, as well as the fraction of filter effluent particles that were detached after deposition. Experimental results indicated that particle detachment is significant beginning from the early phase of filtration. FM removal increased with filter run time, depth and particle size. For each size FM at one filter depth, FM removal increased with filter runtime to a maximum due to ripening and then decreased with filter runtime after ripening due to limited pore space remaining in the filter. The fraction of effluent particles that were detached particles increased with particle size and filter bed depth.     

含水率及坡度对红壤丘陵区崩岗崩壁重力侵蚀影响规律的有限元分析

花岗岩崩岗区崩壁的坍塌、崩落,主要是由于崩壁岩土体存在裂隙,在雨水作用下,由重力作用引发崩壁失去稳态而崩塌,含水率和坡度是影响崩壁重力侵蚀( 稳定性) 的重要因素。为系统探索不同坡度的崩岗崩壁剖面在降雨或干旱环境下的稳定性情况,结合对湖北省通城县室内浸泡或风干不同时间的崩壁土体进行直剪试验得出的强度初始参数,选取 8 种分析坡度,采用 Abaqus 有限单元强度折减法计算了不同坡度的崩壁在不同含水率下的安全系数和潜在滑动面。结果表明: 在同一含水率下,随着崩壁坡度的增大,坡度对临界滑动面位置变化的影响程度减小,安全系数非线性降低; 在同一坡度下,随着崩壁含水率的增大,安全系数先增大后减小,含水率对临界滑动面的影响程度先减小后增大再减小,且影响比较显著。通过数据描点发现安全系数与整体含水率之间存在三次函数关系,与坡度之间呈对数型函数分布,并建立了安全系数与整体含水率和坡度之间的一般定量关系式。综合分析崩壁的破坏方式可知,当崩壁较缓时,崩壁滑动面呈圆弧滑移状,失稳类型属于滑移破坏型; 当崩壁较陡( 尤其＞60°) 时,滑动面近似直线状,失稳属于崩塌破坏型。坡度越大,张拉破坏区向土体内部延伸的深度越大,越易崩塌。通过查阅文献并与前人的研究成果做对比,来验证数值计算结果的合理性与适用性。成果可为崩岗灾害的防治体系建设提供借鉴。